This invention relates to an alignment system and a projection exposure apparatus with the same. The present invention is suitably usable in a lithographic process for the manufacture of semiconductor devices or other devices such as CCD""s or liquid crystal displays, for example, for lithographically transferring or imagewise projecting a pattern of a reticle or mask onto a substrate directly or through a projection optical system in a step-and-repeat or step-and-scan method.
In projection exposure apparatuses for the manufacture of semiconductor devices, for example, increases in density of an integrated circuit have necessitated that a circuit pattern formed on a reticle be projected and printed on a wafer (substrate) with higher resolution.
At the same time, due to miniaturization of a circuit pattern, it is required that a wafer and a reticle having an electronic circuit pattern formed thereon be aligned with each other very precisely. Generally, as an alignment method for the reticle and the wafer, there is a baseline method wherein positional information about an alignment mark provided on a wafer is detected (observed) through an alignment microscope (alignment scope).
This method contains a factor for an error of reticle-to-wafer alignment, called a baseline error, which is an error related to baseline measurement.
FIG. 1A is a schematic view of a main portion of a conventional projection exposure apparatus, and FIG. 1B is a schematic view of a portion of FIG. 1A. The baseline measurement will be described briefly, with reference to these drawings.
In FIGS. 1A and 1B, a reticle 1 is held on a reticle stage 6 by attraction. The projection exposure apparatus is equipped with a reticle reference mark 19 which is positioned exactly with respect to a projection optical system 7 and which is to be used for aligning the reticle 1 with respect to a predetermined position. Second mark detecting means 18 has a detection region at a predetermined position within the projection field of the projection optical system 7, and it serves to optically detect, within this detection region, the positional relation between the reticle 1 and the reticle reference mark 19 as well as the relative positional relation between a second reticle mark 5, provided on the reticle 1, and a second reference mark 14 formed on a substrate stage 11. The second mark detecting means 18 includes moving means. By use of this second mark detecting means 18 and with reference to the reticle reference mark 19, a first reticle mark 4 provided on the reticle 1 is moved into registration with the reticle reference mark 19, and registration is measured. This is called a first measurement. On the basis of the result of this measurement, a deviation between the reticle 1 and the reticle reference mark 19 is detected.
A reference mark plate 12 is provided in a portion of the reticle stage 11, and it has formed thereon a first reference mark 13 which can be detected through first mark detecting means 17 and a second reference mark 14 which can be detected through the second mark detecting means 18. These first and second reference marks 13 and 14 are disposed with a certain interval corresponding to positions of the detecting regions of the first and second mark detecting means 17 and 18. The substrate stage 11 is moved and positioned so that the second reticle mark 5 on the reticle 1 and the second reference mark 14 on the reference mark plate 12 can be detected through the second mark detecting means 18. After such positioning, a relative positional deviation between the second reference mark 14 and the detection center of the second reticle mark 5 is measured, and the deviation is memorized as a deviation of a relative position of the reticle 1 and the substrate stage 11. This is called a second measurement.
Then, a deviation between the detection center of the first mark detecting means 17 and the first reference mark 13 on the reference plate 12 is measured. This is called a third measurement.
From the results of the first to third measurements, the relative distance between the reticle reference mark 19 and the detection center of the first mark detecting means 17 is taken as a baseline, and a relative positional deviation detected by measurement is determined as a baseline correction value.
As described, in conventional projection exposure apparatuses, first the relative position between a first reticle mark 4 of a reticle 1 and a reticle reference mark 19, positioned accurately with respect to a projection optical system 7 for alignment of the reticle 1 with respect to a predetermined position, is detected (first measurement). Second, the relative position between a second reticle mark 5 of the reticle 1 and a second reference mark 14 formed on a reference mark plate 12, provided in a portion of a substrate stage 11, is detected (second measurement). Third, the relative position between a first reference mark 13, having predetected positional relation with the first reference mark 14 on the reference mark plate 12, and the detection center of first mark detecting means 17 capable of optically detecting a mark on the substrate stage 11 and having its detection center positioned at a predetermined distance from the optical axis of the projection optical system 7, is detected (third measurement). From the results of these three detections, the distance between the detection center of the first mark detecting means 17 and the reticle reference mark 19, for alignment of the reticle with a predetermined position, is detected as a baseline, and it is memorized into a storing medium.
The baseline measurement in conventional projection exposure apparatuses is performed with the intervention of a reticle. This creates a possibility that an error of reticle patterning causes a baseline error. It necessitates preparation of reticles to be used exclusively with a particular projection exposure apparatus. This is very inconvenient. In other words, performing baseline measurement through a peculiar reticle to be used with a particular projection exposure apparatus, necessitates that the baseline measurement and stage correction measurement are executed after that reticle is loaded at a predetermined position in the exposure apparatus. This requires complicated operations for exposure apparatus control and for reticle control, and numerous operations have to be involved in the exposure apparatus operation.
Further, the relative distance between the reticle reference mark 19 and the detection center of the first mark detecting means 17 is detected as a baseline length, on the basis of the three measurements made to (i) the reticle 1 and the reticle reference mark 19, (ii) the reticle 1 and the substrate stage 11, and (iii) the substrate stage 11 and the detection center of the first mark detecting means 17. Each of these three measurements may contain a measurement error. Therefore, there is a certain limitation to precision improvement in the baseline measurement. Further, the necessity of three measurements produces a certain limitation to improvement of the baseline measurement speed.
Baseline measurement using a reticle peculiar to a particular exposure apparatus means that the baseline measurement is unattainable if a reticle loaded is different or when no reticle is loaded in the exposure apparatus. This provides a certain limitation to the reduction of operation time for baseline measurement or to baseline measurement control.
Furthermore, baseline measurement or stage running correction measurement cannot be performed if a reticle is placed at a predetermined position within the exposure apparatus. This applies an adverse effect to throughput of the whole exposure apparatus.
It is an object of the present invention to provide an improved alignment system and/or an improved projection exposure apparatus by which a baseline, that is, the relative distance between a reticle reference mark exactly positioned with respect to a projection optical system and a detection center of a first mark detecting means, for detecting positional information related to a first reference mark provided on a substrate (wafer), can be measured with a baseline measurement error factor removed or reduced, such that simplification of baseline measurement control as well as improvement of baseline measurement precision and processing speed are assured, and such that relative alignment of the reticle and the substrate can be made accurately to assure high precision projection and transfer of a pattern of the reticle onto the substrate.
It is another object of the present invention to provide an improved alignment system and/or an improved projection exposure apparatus by which, even if there is no reticle placed on a light path within an exposure apparatus, a baseline, that is, the relative distance between a reticle reference mark exactly positioned with respect to a projection optical system and a detection center of a first mark detecting means, for detecting positional information related to a first reference mark provided on a substrate (wafer), can be measured with a baseline measurement error factor removed or reduced, such that simplification of baseline measurement control as well as improvement of baseline measurement precision and processing speed are assured, and such that relative alignment of the reticle and the substrate can be made accurately to assure high precision projection and transfer of a pattern of the reticle onto the substrate.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.